When Our Big Brother Became Our Little Brother
by Olv1993
Summary: After an argument occurred at the Matsuno household one night, Osomatsu went to seek help from Dr. Dekapan, however it backfired and now he's a toddler.
1. When Big Brother Disappeared For A Day

Wow it's been a loooooooong time since I've wrote anything, even longer since I've uploaded something. I've really gotten into Osomatsu-San lately and finally got inspired to write some fics again, so this won't be the last thing I write. This one in particular was inspired by an RP with a friend, it was just so cute. ;-;

Things in square brackets are in English.

* * *

Karamatsu woke up feeling refreshed after a good night's rest. He sat up and stretched.

"Morning [brothers]! It's time to wake up as the sun smiles upon us!" He smiled, looking at his siblings, hoping they felt as refreshed as he did.  
"...Let me sleep nii-san...Hm...That's odd...The futon feels more spacious?" Todomatsu said sleepily and proceeded to roll away from Karamatsu in order to see how much room he had gained overnight. "Eh? Where's Oso-nii?"

The others then looked at the spot where Osomatsu usually slept, he really wasn't there which was unusual considering the fact that they usually got up together.

"Eh!? Where's nii-san!? Did you shrink!? Are you under the covers!?" Jyushimatsu sat up quickly and pulled the covers off everyone. "AHH! Nii-san's not here!? He left early!"  
"That's weird..." Choromatsu mumbled quietly, sitting up. "It's not like Osomatsu to be up before everyone."  
"He probably snuck out to play pachinko and take our money before we woke up or maybe there's a horse race on today?"  
Todomatsu sighed, he really hoped that wasn't the case as he earnt that money himself, but still it was hard to think of any other reason for Osomatsu to not be around so early in the morning.

That got everyone up, it was just like him to take their money so he could gamble, like the one time they were all sick and instead of taking care of them, he took their money, hoping to win and make them feel better, although he probably would have kept those winnings for himself, who knows why he'd think that would work. The five remaining Matsu brothers swiftly moved to where they kept their wallets and found that they were all still there, including Osomatsu's. They all immediately knew something was off, just what was Osomatsu doing out so early? They got dressed and went to have breakfast, he wasn't there either. They searched the whole house for him, Osomatsu had left the building, his whereabouts completely unknown.

Seeing as there was no clue to what Osomatsu was doing up so early, the five brothers just decided to get on with their day, they'd just have to ask him what he was doing later when he got home. Hours went by and the brothers ended up eating lunch minus the eldest. It was about 2.40pm when they were all chilling in the living room, feeling bored when Karamatsu spoke up.

"Hmm...It seems Osomatsu hasn't come back yet, that's strange..."  
"He's probably playing pachinko or at the horse races." Todomatsu repeated his thoughts from earlier, while texting on his phone.  
"Totty, he left the house without any money! There is no way he could be at either of those places." Choromatsu frowned at him in frustration, then sighed. "I just hope he hasn't gotten himself into trouble..."

Before anything else could be said, the house phone rang. The brothers looked at each other hesitantly, maybe Osomatsu really had gotten into trouble and this was someone calling them to let them know of the bad news.

"Maybe...That's him calling now?" Todomatsu tried to sound positive, though they all knew it obviously wasn't Osomatsu, himself, he had no mobile phone of his own nor did he take any money to use a phonebox.  
"Karamatsu, get the phone." Choromatsu ordered, looking back at his computer screen. He was worried he'd jinxed it with what he'd just said, but he also didn't want to miss out the new Nyaa-chan merch he was waiting for.  
"Yes, my [brother]! I'm sure that it's nothing to worry about!" Karamatsu got up, hoping his words eased the tension, but the glances between them made it clear they either didn't believe him or simply weren't listening as usual.

Karamatsu brushed it off, thinking they were just struggling to believe this phone call was anything but something bad about their eldest brother. He almost hesitated to pick up the phone himself, unable to also feel a small amount of dread.

"Hello, Matsuno residence, how may I help you dear caller?"  
"Ah, hello which of the Matsuno brothers is this, dasu?" Dekapan responded, Karamatsu knew it was unusual for him to call.  
"Karamatsu! Do you need Jyushimatsu's assistance? I can go and get him?" "Hello Karamatsu, No, no. I think you may actually be the best one to talk to about this, um...I'm calling because a slight incident occurred with Osomatsu, dasu."  
"Huh? Osomatsu? What happened!?" He hadn't realised it, but Karamatsu's voice was loud, his worry for the eldest clear, his usual facade broken.  
"Sounds like Kara-nii may be talking to Oso-nii..." Todomatsu said, his tone uncaring, clearly more interested in his phone.

Choromatsu looked at his youngest brother with a frown again.

"Pretty sure it's more likely to be someone calling about Osomatsu, there's no way he could call us on his own."  
"You don't think he's..." Ichimatsu mumbled quietly.  
"No, no! I'm sure he'll be ok! He always bounces back from whatever happens." Choromatsu tried to reassure them all.

"He's not hurt exactly, though he is rather upset dasu. You and your brothers don't need to worry he's safe. However, um..." Dekapan hesitated, unsure of how to put it. He could tell that the young man was worrying more than he let on.  
"Please...Tell me what's happened to him." Karamatsu whispered, his words only just being picked up by the elder man.  
"Well...To put it simply...Osomatsu is now a toddler."

Karamatsu widened his eyes in surprise, unsure if he heard him properly. So, in a way he was the eldest now? Was that right? That's going to be something that'll take time to get used to, that is if he isn't changed back soon.

"...Can you please repeat that?"  
"Osomatsu Matsuno, your eldest brother...Was turned into a toddler."  
"[Oh my God]! How did that happen!?"  
"...Things. I'm sure you are aware of the kind of things that could happen at my lab. Could you please come and collect him? I think it would be best for Osomatsu to explain himself. Right now I think he needs his brother's company. Unfortunately I do not have a cure right now, so someone needs to watch over him dasu."  
"[Of course]!" Karamatsu posed, even though he could not be seen, trying to keep up his cool facade. He somewhat hoped that if the others were listening, their worries would be relieved.  
"Come quickly dasu." Dekapan responded, then hung up.

Karamatsu hung up the phone and poked his head through the door to where his younger brothers were waiting for the news.

"That was Dr. Dekapan...He requested for me to go and collect Osomatsu."  
"Eh!? That's a weird place for him to go...What was he doing there!?"  
"I think it's best we explain later, but [no worries] for he is safe!" Karamatsu smirked, which made Todomatsu sigh.  
"I guess he must be if you're sticking to that painful facade..."  
"[Farewell]! We will be returning soon!" Karamatsu left the house, his confident smile turning into a worried frown.

Did he really hear correctly? No way could that be true, right? Surely Osomatsu had just got him in on some kind of prank. He knew Dekapan had a lab full of unusual, yet amazing things from personality altering caps to medicine that could allow cats to talk and understand a human's true intentions, but Osomatsu turned into a toddler? That just seemed too farfetched, why would Osomatsu want to do that? But he couldn't ignore the request to go to the lab, maybe something really was wrong?

Soon he found himself at Dekapan's lab, he supposed getting lost in thought allowed time to pass quickly. He entered the lab and posed in one of his usual painful poses, hoping to reassure all who were there.

"I have arrived!"  
"Painful." Was the response he heard, from a vaguely familiar voice.

Karamatsu looked at the source, there he was. Osomatsu stood there before him in nothing but his usual hoodie with their family symbol on it. Dekapan hadn't lied, Osomatsu had turned into a young child no more than 3 or 4.


	2. Osomatsu's Dilemma

I had this chapter written up not long after I had wrote the first one. But due to struggling with the third one, I held it off, then sorta didn't get around to writing more. But since I have been doing some writing lately, I thought I'd post the second chapter up too and hopefully inspiration will hit me and the third one will be done too. Thank you for your patience.

* * *

Osomatsu frowned at his brother's stare, this whole situation was embarrassing enough without being looked at like some freak of nature. Then again being a sextuplet, he supposed they could be considered one.

"Now now, [brother]..." Karamatsu walked up to him and picked Osomatsu up, giving him his best reassuring smile. "I have come to take you home."

Osomatsu panicked and smacked Karamatsu's cheek. He yelped in pain, but did his best to be careful not to drop him.

"Owwww! W-what are you doing Osomatsu!? That hurt!" He frowned worriedly, he was only trying to make him feel better about his predicament.  
"P-put me down!" Osomatsu demanded.  
"...I just thought you might not want to walk home [dear brother]."  
"...N-no." He had to admit Karamatsu had a point, the walk home would surely feel a lot longer and more tiring now his legs were a lot shorter, but that wasn't the main issue here.  
"Hm? Does that mean you'll let me carry you then?"  
"...At least let us...Swap hoodies first." Osomatsu hesitated with his suggestion, feeling that even Karamatsu may refuse, or he'd have to explain himself which was going to be embarrassing. Then again, he was probably going to have to tell him anyway.  
"Huh? Why?" Karamatsu raised an eyebrow.  
"I don't want people seeing me...Like this." He looked at the floor nervously, clearly this who ordeal was embarrassing for him.

Karamatsu nodded and set Osomatsu down, then took off his hoodie, quickly swapping with him. He knew he was right to ask him to come instead of one of the others. Anyone else and they would have laughed at his tiny form, anyone else and they probably wouldn't have swapped hoodies so the others would think that Osomatsu had tricked them into taking something to age them into a young child. But not Karamatsu, he always tried to make sure his brothers knew he cared, even if he'd overdo it like climb a mountain to get fresh water instead of simply getting it from the tap, but what mattered is that he was willing to help him, no sarky comments or questions asked.

"Better now?" Karamatsu smiled.  
"Yeah..." Osomatsu replied, looking at him and reaching his hands up, wanting to be picked up.

Karamatsu's smiled brightened as he couldn't help but feel the way Osomatsu was acting was simply adorable. He couldn't resist picking him up again and cuddling him, then he blinked and looked at him curiously.

"Ah...Where are your jeans and shoes? We should at least put your shoes on."

Osomatsu was hesitant to respond, he was embarrassed about what had happened, but pointed to them anyway. They were soaked, Osomatsu couldn't bare to look at the expression on his brother's face.

"W-WHAT HAPPENED!?" Clearly Karamatsu's face must have been shocked to respond in such a way, he had hoped he wouldn't respond in that way and shrug it off.  
"...Things. I don't want to talk about it."  
"[Brother]? Did you wet yourself?"  
"N-no!" Osomatsu finally looked at him, clearly upset that he'd even suggest he'd do such a thing. Sure, he was a child, but he wasn't one on the inside, he'd never start doing such a thing again or so he'd like to believe.  
"It's ok [brother]!" Karamatsu gave him a reassuring smile, the one he would have preferred to have been given in the first place.

Karamatsu looked around for a bag to put them in, realising Dekapan wasn't around, he'd probably gone to another part of the lab to record the results of whatever Osomatsu took or maybe he'd even started working on a cure. He found a plastic bag and some gloves, then put Osomatsu back down as he put the wet clothes in it.

"How did you become a child anyway?" He asked while sorting Osomatsu's clothes out.  
"...I wanted to become older...So I asked Dekapan to help." Karamatsu hesistated and looked at him questioningly.  
"Huh? Why would you want that?"

Sure being sextuplets had disadvantages, they all had the same face, one of them gets into trouble, they all would. It was easy to pin the blame of something on each other and not many people would believe there was six of them without seeing them all together. Having five other brothers the same age of you could also be seen as having five other rivals when it came to things such as finding a date. They could never really have a moment to themselves either. But they had advantages too, their sextuplet dynamic never got old. Once it even helped them get a job, they just had to switch shifts every so often so none of them had to work the whole time. Of course, their boss found out and fired them as they couldn't afford to pay six people for one job, but it was a good plan while it lasted. They also had five friends who were always there for them, Osomatsu always had a way to convince them to join in on his schemes. It was never lonely or boring when they were together.

"...It's because you guys kept saying I'm not really the older brother." Osomatsu's face was serious, his eyes sad.  
"...Ah..." Karamatsu remembered now, he understood, he hadn't realised it had affected Osomatsu so much.

* * *

Last night they were hanging out at home as usual. Todomatsu was on his phone, Jyushimatsu and Ichimatsu playing with a cat, Choromatsu on his laptop, Karamatsu checking himself out in the mirror, Osomatsu reading some manga.

"What're you looking at Choromatsu nii-san?" Todomatsu asked, looking at his laptop screen curiously.  
"Hm? Just people discussing this anime that has triplets in it."  
"Ohhh?" Todomatsu quickly became disinterested. He still read some from time to time, but had mostly outgrown it from when he was a kid, unlike Choromatsu, who was often up to date on the latest manga and anime.  
"Someone here is saying that in the west the firstborn out of triplets is the eldest while the last one to be born is the youngest, unlike here where there's the tradition of the eldest being the last to be born, youngest being first.  
Which would make the characters age-related dynamics different if they were Westerners."  
"W-what!? Do you think that's true for sextuplets too!?" This had brought Todomatsu's attention back, if this was right, then wouldn't that make him the oldest brother?  
"I don't know, maybe? Guess..I'd be the fourth son?"  
"Heh. Either way I'm still the disappointment." Ichimatsu glanced over a them, a slightly amused look on his face.  
"The only disappointment is telling my Karamatsu [Girls] that I am actually the fifth, not second son." Karamatsu chimed in, with a poor attempt to make Ichimatsu feel any better about himself, receiving a glare from said man.  
"I'm Jyushimatsu! Second son of the Matsuno family! But I'm gonna hit a [number one home run]!"  
"It's about time you guys looked up to me. After all, I'm the only one who can get a foothold on society." Todomatsu smirked, he was so proud of himself, maybe they'd all finally start listening to him. No more ganging up on him as the youngest brother.  
"Eh? You're all joking right? I'm the eldest brother, that's how it is. Always has, always will." Osomatsu frowned.  
"You're just jealous that you're not really the oldest. Don't be such a baby Osomatsu ni-..." Todomatsu smirked more, taking off the usually honorfic.  
"Us big brother's will take good care of you baby Osomatsu!" Jyushimatsu smiled, oblivious to the atmosphere.  
"It'll be weird having Jyushimatsu nii-san as my big brother, but I can deal with that." Ichimatsu added.  
"I guess it's not a huge difference for me." Choromatsu shrugged. "Though maybe you 'older' brothers will listen more?"  
"[Brothers]? I'm sure you'll all take good care of us younger ones." Karamatsu smiled, so supportive of the others.  
"Oh come on! One person on the internet says something and you all go along with it!? Are you all idiots!? Really!? This isn't funny! This is Japan and that means whichever order we were born in, it still makes me the oldest!? You really want someone like Todomatsu leading us!? He wouldn't know fuckall what to do because he's the big baby here! Changing our birth order would change everything! This joke is so terrible it's not even funny! I'm the big brother here, you can't just change that suddenly on a whim! I'm the eldest, Karamatsu is the second son, Choromatsu the third, Ichimatsu the fourth, Jyushimatsu the fifth and Todomatsu is the youngest! That's how it is and that's final!" Osomatsu stood up and yelled at them, after finishing his rant, he stormed out.  
"Geez, we were only having a little fun, we knew that really..." Todomatsu rolled his eyes, but the eldest brother had already left, while the others sat there in an awkward silence.

Later when they'd seen Osomatsu he appeared to be over it, acting like his regular self. No one mentioned anything about the things that had happened earlier and Karamatsu felt relieved, thinking things were back to normal...But they weren't.

* * *

Osomatsu put his hands over his face as if to hide from his embarrassment. Karamatsu looked at him sadly, feeling awful about being oblivious to his brother's true feelings.

"I'm sorry, I should have realised how much it hurt you and stopped the others..." Karamatsu apologised.  
"Shut up." Osomatsu frowned.  
"But..." Karamatsu gripped onto the bag he was holding, wanting to say more, but decided it was best not to. "We should say goodbye to Dr. Dekapan before we leave, where is he?"

Osomatsu pointed to the room, so Karamatsu walked over to the door and knocked in.

"Come in dasu!"

Karamatsu opened the door, but stayed where he was.

"We just came to say goodbye before we left."  
"Thank you for coming to pick him up, I'm sorry about what happened, it make take a while to find a way to reverse the effects, but I'm sure he'll be in good hands until then. I'll call the house if there are any developments dasu."  
"Thank you, see you soon."  
"Goodbye young Karamatsu, young Osomatsu."

The brothers left the lab quietly, Osomatsu clinging to his brother as he was held in his arms, nervous about how the others would react. At least they'd hopefully think it's Karamatsu instead, but could he pull it off? Sure Karamatsu had been in the drama club and that probably helped him become a decent actor, but Osomatsu had only ever pretended to be his brothers while pranking people when they were younger. Acting like each other had gotten harder and harder over the years as they became more unique, people found it a lot easier to tell who was who compared to when they were ten year olds. Though, maybe if he tried acting like they did around the age he was now? Maybe that would make things more believable? But he didn't really want to be treated like a little kid, he was still their elder brother mentally.

"Oh how cute!" That snapped Osomatsu out of his thoughts as a girl was looking over at them.  
"Hm?" Karamatsu glanced at her, tempted to flirt, but then again, he had to get his brother home.  
"Is he your child?" She asked while walking over to them.  
"A-ah, no, no! He's my [brother]!" Karamatsu, smiled, feeling a little awkward but doing his best to seem like he was a cool older brother carrying his baby younger brother...Even if he was really his older brother turned into a child.  
"That's so sweet of you to be taking care of him, you're such a good big brother to be looking after him on your day off work. I'm sure you'd be such a great father one day!"  
"T-thank you." Karamatsu replied, not having the heart to correct her.  
"It would be nice to get to know someone like you." She smiled and fumbled around in her bag, then she pulled out a pen and some paper, proceeding to write something down. "Here, take my number! Call me!"  
"[Thank you] Karamatsu [Girl]." Karamatsu took the paper and put it in his pocket, while Osomatsu frowned at the girl as she walked off. "...Hm? What is it [brother]?"  
"..." Osomatsu looked away from him, clearly annoyed, maybe even a little jealous about what had just happened between Karamatsu and the girl.  
"...You can talk to me. What's wrong Osomatsu?"  
"Shut up!"  
"There's no need to be jealous [brother]. She gave her number to me, but right now, I look more like you. So, I suppose they would think I'm you."  
"Painful..."

Karamatsu couldn't help but smile a little at the response and continued to walk them home.


End file.
